Orchard CMS contentfields vs class properties - orchardcms

Question

When should one use Fields from the CMS and when to use class properties and database fields?
My scenario:
Created a product content part with fields usage (text) and price (double). Then I have created a contenttype product and added the part.
I could also have created a product record in models and added a product table. And a part with driver etc.
At first glance I dont see difference besides option 2 requiring programming.
However I did encounter a problem:
In option 2 I could use repository and create a lot of items programmatically.
In option 1 I could create product content items but was not able to fill in the fields of the product part (no errors, but fields remained empty)
So when to use option 1 and when option 2?
And is my problem with option 1 related to that option?
EDIT: Clarifiation of problem with option 1
I have created a productpart. To the productpart I have added the field price which is a decimal and I have added a field Usage of product which is a text field.
In the code I have the following:
dynamic item = _cm.New("Product");
item.TitlePart.Title = "Mijn dummy product";
item.BodyPart.Text = "Some dummy text for this product";
item.ProductPart.Price.Value = new decimal(20.5);
item.ProductPart.Usage.Value = "Some dummy usage of this product";
_cm.Create(item);
After running the code, the product is created with the correct title and body text, but Usage and Price come up emtpy.
I also tried it with the item.As<> method. But that does not compile to As since I have not created an object with that name.

Answer 1

A difference is that fields can only be queried against with projections but more generally parts and fields are for different usage: parts are meant to be reusable, not fields. Price for example is going to be a characteristic of any product so it should probably be a part property. Usage, I don't know, it sounds fairly specific to what you're doing and may be better done as a field.
As for your problems with option 1, it's hard to say without seeing your code but chances are you're doing it wrong.

Answer 2

Apparently the item has to be created first before accessing the contentpart and fields. At least it worked for me.
Please verify if my analysis is correct. If so I will mark it an answer, as well as the answer of Bertrand.
The code now looks like this:
var item = _cm.New("Product");
item.As<TitlePart>().Title = "Mijn dummy product";
item.As<BodyPart>().Text = "Some dummy text for this product";
//Create first before accessing the contentparts that are dynamic.
_cm.Create(item);
//And then access the product via dynamic object.
dynamic dyn = item;
item.ProductPart.Price.Value = new decimal(20.5);
item.ProductPart.Usage.Value = "Some dummy usage of this product";